The present invention relates to a switched mode power supply apparatus, and more particularly, it relates to a high-breakdown voltage semiconductor switching device used in a switched mode power supply apparatus for repeatedly switching a principal current.
Recently, it is regarded significant to reduce the standby power of electric home appliances from the viewpoint of preventing the global warming, and there are increasing demands for a switched mode power supply apparatus consuming smaller power in a standby mode.
Now, a conventional switched mode power supply apparatus will be described.
FIG. 22 shows an exemplified circuit configuration of the conventional switched mode power supply apparatus. As shown in FIG. 22, the conventional switched mode power supply apparatus includes a primary rectifying/smoothing circuit 111, a main circuit 112, a transformer 104 and a secondary rectifying/smoothing circuit 121.
Specifically, an AC voltage input between input terminals 116 and 117 of the primary rectifying/smoothing circuit 111 is rectified and smoothed by the primary rectifying/smoothing circuit 111 so as to be supplied to the main circuit 112 as an input DC voltage. At this point, the primary rectifying/smoothing circuit 111 includes a diode bridge 131 and an input capacitor 132, so that a voltage having been full-wave rectified by the diode bridge 131 can be smoothed by the input capacitor 132 for supplying the resultant voltage to the main circuit 112.
The main circuit 112 includes a semiconductor switching device 113 and a voltage control circuit 114. The semiconductor switching device 113 and the voltage control circuit 114 can be integrated on one chip. The transformer 104 includes a primary winding 141, and the primary winding 141 and the semiconductor switching device 113 are serially connected to each other, and the input DC voltage is supplied from the primary rectifying/smoothing circuit 111 to this serially connected circuit.
The control terminal of the semiconductor switching device 113 is connected to the voltage control circuit 114, so that the semiconductor switching device 113 can be controlled to be turned on/off in accordance with a gate signal output by the voltage control circuit 114.
The transformer 104 further includes a secondary winding 142 magnetically coupled to the primary winding 141 and a bias winding 143 magnetically coupled to the primary winding 141 and the secondary winding 142. When the semiconductor switching device 113 is switched and a current intermittently passes through the primary winding 141, a voltage is induced in the secondary winding 142 and the bias winding 143.
The second rectifying/smoothing circuit 121 generates a DC output voltage by rectifying and smoothing the voltage induced in the secondary winding 142 and outputs the DC output voltage from output terminals 126 and 127. Specifically, the secondary rectifying/smoothing circuit 121 includes a diode 122, a choke coil 123 and first and second output capacitors 124 and 125. The choke coil 123 and the first and second output capacitors 124 and 125 are connected to one another in a π shape, and the voltage induced in the secondary winding 142 is half-wave rectified by the diode 122 and is smoothed by the choke coil 123 and the first and second output capacitors 124 and 125.
A voltage generated on the both ends of the bias winding 143 is input to the control terminal of the semiconductor switching device 113 through the voltage control circuit 114. In other words, the switched mode power supply apparatus of FIG. 22 employs a ringing choke converter (RCC) system, and the semiconductor switching device 113 performs a switching operation in a self-excited manner by using the voltage generated in the bias winding 143.
A voltage between the output terminals 126 and 127 is fed back to the voltage control circuit 114 through a photo coupler 129. For example, in the case where the voltage between the output terminals 126 and 127 is lowered, the voltage control circuit 114 forcedly increases an on period of the semiconductor switching device 113, and on the contrary, in the case where the voltage between the output terminals 126 and 127 is increased, the voltage control circuit 114 forcedly reduces an on period of the semiconductor switching device 113. Thus, the voltage appearing between the output terminals 126 and 127 is kept at a given value.
Within the voltage control circuit 114, an auxiliary DC voltage is generated by using the voltage induced in the bias winding 143, and therefore, the voltage control circuit 114 is operated by the auxiliary DC voltage except for the starting time of the switched mode power supply apparatus.
In the starting time of the switched mode power supply apparatus, namely, when the AC voltage is applied between the input terminals 116 and 117, the semiconductor switching device 113 is not in a switching operation, and hence, no voltage is induced in the bias winding 143 and no power is supplied to the voltage control circuit 114. Accordingly, in order to start the switching operation of the semiconductor switching device 113, a low voltage sufficient for activating the voltage control circuit 114 is supplied from the primary rectifying/smoothing circuit 111 through an externally provided resistance 151 (with a high breakdown voltage and high power).
In the aforementioned switched mode power supply apparatus, loss is mainly caused in the semiconductor switching device 113. Generally, a MOSFET (metal oxide semiconductor field effect transistor) is used as the switching device 113. In general, switching loss caused in turn off is large in a bipolar transistor but the switching loss is small in a MOSFET because its switching speed is high. In contrast, a MOSFET has large conducting resistance differently from a bipolar transistor and hence its conducting loss cannot be ignored. Accordingly, when a large current passes through a MOSFET, large loss is caused.
Recently, also in the technical field of switched mode power supply apparatuses, an attention is paid to a bipolar type IGBT (insulated gate bipolar transistor) obtained by implanting minority carriers into a drift layer as compared with a unipolar type MOSFET. In the conventional switched mode power supply apparatus of FIG. 22, if an IGBT is used as the switching device 113, although the conducting resistance is smaller because conductivity modulation is caused as in using a bipolar transistor, the switching speed is lowered and the switching loss is increased because minority carriers are used.
In the switched mode power supply apparatus of the RCC system described above, in the case where a load connected to the output terminals 126 and 127 is heavy, the switching frequency of the switching device 113 is lowered and an on period of the switching device 113 is increased, and as a result, a large current passes through the primary winding 141 so as to keep the voltage between the output terminals 126 and 127 at a given value. On the contrary, when the load is light as in a standby mode, the switching frequency of the switching device 113 is increased and an on period is reduced, and as a result, the current passing through the primary winding 141 is reduced so as to keep the voltage between the output terminals 126 and 127 at a given value.
Accordingly, comprehensively considering both of the switching loss and the conducting loss, when the load is heavy, the frequency is lowered and the current is increased, and therefore, a MOSFET is disadvantageous and an IGBT is advantageous. On the contrary, when the load is light as in a standby mode, the frequency is increased and the current is reduced, and therefore, a MOSFET is advantageous and an IGBT is disadvantageous.
FIG. 23 is a diagram for showing the comparison, in the relationship between load and loss, between a MOSFET (of a lateral type having a drift region with a resurf structure) and an IGBT (of a lateral type) used in a switched mode power supply apparatus. As shown in FIG. 23, when the output is lower (the load is lighter), the switching frequency is increased and hence the loss caused in the IGBT is larger, but when the output is higher (the load is heavier), the switching frequency is lowered and hence the loss caused in the MOSFET is larger.
Patent Document 1: Japanese Laid-Open Patent Publication No. 7-153951
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-345242
Patent Document 3: Japanese Patent Publication No. 6-52791 (U.S. Pat. No. 5,072,268)
Non-patent Document 1: D. S. Byeon et al., The separated shorted-anode insulated gate bipolar transistor with the suppressed negative differential resistance regime, Microelectronics Journal 30, 1999, pp. 571-575